Examples of a semiconductor memory device (hereinafter simply referred to as a memory device) include a DRAM and an SRAM, which are categorized as volatile memories; a mask ROM, an EPROM, an EEPROM, a flash memory, and a ferroelectric memory, which are categorized as non-volatile memories; and the like. Most of these memories formed using single crystal semiconductor substrates have already been put into practical use. Among the semiconductor memories, flash memories become available on the market and are mainly used for portable storage media such as USB memories and memory cards. The reason for this is that flash memories resist physical shock and can be conveniently used because they are non-volatile memories which can repeatedly write and erase data and can retain data without supply of power.
As flash memories, there are a NAND flash memory in which a plurality of memory cells are connected in series and a NOR flash memory in which a plurality of memory cells are arranged in matrix. Each flash memory has a transistor which functions as a memory element in each memory cell. Further, the transistor which functions as a memory element has an electrode for accumulating electrical charge that is called a floating gate between a gate electrode and a semiconductor film serving as an active layer. Accumulation of electrical charge in the floating gate enables data retention.
References 1 and 2 each disclose a thin film transistor including a floating gate formed over a glass substrate.